Electric quantity memory element

ABSTRACT

An electric quantity memory element comprising a first electrode made of a metal electrochemically dissolved or deposited in accordance with Faraday&#39;&#39;s laws, a second electrode made of a valve metal and an aqueous solution containing a water soluble salt of the metal of the first electrode as an electrolyte.

United States Patent Sekido et al.

14 1 Ju1y25,1972

[541 ELECTRIC QUANTITY MEMORY 3,052,830 9/1962 Ovshinsky ....317/231 ELEMENT 3,423,642 1/1969 Plephal et al. ...317/231 3,423,648 l/1969 Mintz .317/231 [72] Inventors: Satoshi Sekido, Kyoto; Tomohiko Arita, 3 512 9 5 9 0 Haberman et aL 317/230 Osaka v 3,158,798 11/1964 Sauder ..317/231 [73] Assignee: Matsushita Electric Industrial Co., Ltd, ,423,643 1/1969 Miller ..317/231 Osaka, Japan 3,275,901 9/1966 Merritt et a1 ..317/230 [22] Filed: 1970 Primary Examiner-James D. Kallam [21] Appl. No.: 72,486 Attornqv-Stevens, Davis, Miller & Mosher Foreign Application Priority Data [57] ABSTRACT An electric quantity memory element comprising a first elec Sept. 20, 1969 Japan.....- ..44/75564 trode made of a metal electrochemically dissolved or [52] U 8 cl 317/231 317/230 340/173 CH deposited in accordance with Faraday's laws, 21 second elec- [51] imlg 9/16 G1 l c 13/02 trode made of a valve metal and an aqueous solution contain- [58] Field of Search ..17 230 231 a water sluble first electmde as Y electrolyte. [56] References Cited UNITED STATES PATENTS 9 Claims, 18 Drawing Figures 2,710,369 6/1955 Booe ..317/230 CELL CELL (v) QLBRENL A (mA/cm IO (0) Mo,T1 40 (mA/cm (ELECTRICITY WHEN DISSOLVED ELECTRICITY WHEN DEPOSITED PATENTEJuL25 I972 3.679345 sum 01 or CELL CELL (mA/cm 5'0 |OO 0 (ELECTRICITY WHEN DISSOLVED ELECTRICITY WHEN DEPOSITEDWIOO PAIENTEDJIJL 25 I972 SHEET 0U [1F 10 FIG. 4

CONC OF POTASSIUM SODIUM TARTRATE (g/L) Cu(BF4)2 350 g/l PATENTEDJUL25 m2 SHEET 05 HF 10 FIG. 5.

0 6L :2: mmnhwrse.

CONCOF PHOSPHOKIC ACID (g/l) A93 Pq IOOg/I FAIENTEBJUL 25 I972 sum 070F102 IQOOO' I60 260 ELECTRICITY (mAh) IOOO' PATiNTl-immzs um I 3.679 @945 sum near 10 FIG. l2

{START SIGNAL OOMPLEHON SIGNAL COMPLETION SIGNAII.

PATENTEB JUL 25 m2 sum 09 0F 10 FIG., l3

FIG.

FIG.

P'ATENTEI'lamzs m2 SHEET 10 0F 10 wild, Ill? 1 4 4 a 4 6 VIII/I, ll/l/ I'Ha) FIG.

FIG. I7(b) TIME ELECTRIC QUANTITY MEMORY ELEMENT This invention relates to an electric quantity memory element comprising a first electrode made of a metal electrochemically dissolved or deposited in accordance with the Faradays laws, a second electrode made of a valve metal and an aqueous solution containing a water soluble salt the metal of the first electrode as an electrolyte.

The electric quantity memory element according to the present invention can be used to integrate and register the quantity of electric charge carried by a metal transferred from the first electrode to the second electrode by utilizing the current interruption phenomenon which takes place when the quantity of the metal of the first electrode, which has been deposited on the second electrode during the transfer of electric charges from the first electrode to the second electrode, is completely dissolved away by making a current to flow in the opposite direction. It is also used for the indication of a time interval, during which a constant current is made to flow, the alternating transfer of a constant quantity of an electric charge and the integration of the portion of a current above or below a predetermined value.

There have heretofore been proposed various coulometers utilizing the electrochemical dissolution and deposition of such metals as copper, silver, mercury and lead in accordance with the F aradays laws. These coulometers are mostly used to determine the quantity of electricity from the measurement of the change of the weight or volume of the electrode metal electrochemically dissolved or deposited. In order to use these coulometers for controlling a machine in accordance with the electrical quantity thus determined, it is desirable to be able to produce an electric signal directly from the change of the electrical quantity.

To the end of deriving an electric signal directly from the change of an electrical quantity, there has been proposed an apparatus comprising a first electrode of such a metal as silver electrochemically dissolved or deposited according to Faraday s laws, a second electrode of such an inactive metal as platinum and gold and an aqueous solution containing a water soluble salt of silver as an electrolyte or a solid electrolyte such as Agl, AgBr, Ag SI, Ag SBr, PbAg I KAg,l etc, In this apparatus, when silver which has been deposited on the second electrode is completely re-dissolved by causing the same quantity of electric charges as that which has been transferred during the deposition of silver to be transferred in the opposite direction, a different kind of reaction takes place at the second electrode to cause a change of the potential of the second electrode. From this change, the electric quantity transferred during the deposition of silver may be known. The different kind of reaction as mentioned above means the electrolysis of water for electrolyte of an aqueous solution or the electrolysis of the solid electrolyte when it is used as the electrolyte. When this reaction takes place, reaction gases produced or other reaction products give rise to various problems. In this respect, consideration should be paid to avoid the increase of the cell voltage separately in each case. The increase of a cell voltage should be held below 1.2 volts in the case of a liquid electrolyte and below 0.8 volt in the case of a solid electrolyte. Because of this limitation on the cell voltage, a large change of the cell voltage can not be expected when the deposited metal has been completely re-dissolved.

An object of the present invention is to provide a repeatedly usable memory element for storing a quantity of electricity, in which a valve metal such as tungsten and tantalum is used as the material of the second electrode to be plated with the metal electrochemically dissolved and deposited, and which can provide a large voltage change or effect a remarkable current interruption operation at the time of the complete dissolution of the deposited metal from the second electrode so as to be suitable for use as a means to produce an electric signal in response to the change of an electrical quantity.

The present invention will now be described with reference to the accompanying drawings, in which:

FIG. I shows the cell voltage (solid curve) and the cell current (dashed curve) versus the ratio of the electricity for the dissolution to that for the deposition of copper or gold, Graph (:1) being obtained by using molybdenum of titanium for the second electrode and Graph (b) being obtained by using tan talum for the second electrode;

FIG. 2 shows the ratio of the electricity for the dissolution to that for the deposition of the metal of the first electrode versus the number of cycles of the alternating transfer of the metal of the first electrode with the parameters of various materials sealing the second electrode;

FIG. 3 shows the limit of concentration of Cu or Ag ions (solid curve) and the lower limit of the electrolyte temperature (dashed curve) versus the hydrogen ion concentration of the electrolyte;

FIG. 4 shows the relation between the lower limit of the electrolyte temperature and the concentration of potassium sodium tartrate in the electrolyte;

FIG. 5 shows the relation between the lower limit of the electrolyte temperature and the concentration of phosphoric acid in the electrolyte;

FIG. 6 shows the relation between the lower limit of the electrolyte temperature and the concentration of methanol in the electrolyte;

FIGS. 7 and 8 are schematic longitudinal sectional views showing the construction of preferred embodiments of the electrical quantity memory element according to the present invention;

FIG. 9 shows the performance of the electrical quantity memory element of FIG. 7;

FIG. I0 is a block diagram showing a meter using the electrical quantity memory element according to the present invention;

FIGS. 11 and 12 are circuit diagrams showing other meters using the electrical quantity memory element according to the present invention;

FIG. 13 is a circuit diagram showing a battery charger using the electrical quantity memory element according to the invention;

FIGS. 14, I5 and I6 are schematic longitudinal sectional views showing other embodiments of the electrical quantity memory element according to the present invention;

FIG. 17a shows a circuit diagram employing the electrical quantity memory element according to the present invention for integrating electric signals above a threshold value; and

FIG. 17b shows a circuit diagram employing the electrical quantity memory element according to the present invention for integrating electric signals below a threshold value.

The oxide formation potential of such valve metals as tungsten, tantalum, titanium, niobium, molybdenum and aluminum is lower than (i.e., electronegative to) the dissolution or deposition potential of such metals as copper, silver and lead. Therefore, if these valve metals are used for the aforementioned second electrode, copper, etc., will be deposited on an oxide film formed on the second electrode. Therefore, it often occurs that copper or the like metal is deposited in a poor state.

Also, in the presence of an oxide film on the electrode, the impedance of the electrode is extremely increased and the cell current is reduced to a neglibible value. For this reason, heretofore, these valve metals have seldom been used for the electrochemical deposition of metals thereon.

However, it has been found experimentally that the impedance of the electrode of a valve metal such as molybdenum, tungsten or tantalum is extremely reduced if the electrode is plated with such a metal as copper, silver or lead, that the impedance is suddenly increased when the plated metal is completely dissolved off the electrode, and also that the latter metal is deposited on the former metal with good adhesion. The reason why the impedance is decreased in the presence of the deposited metal is not clear. At any rate, by using the above valve metals in place of platinum or gold, it is possible to provide the effect of sharply increasing the impedance of the electrode to substantially interrupt the cell current at the time when the deposited metal is completely re-dissolved off the electrode. Also, an extremely high cell voltage of the order of 25 to 50 volts may be obtained with a negligible value of the cell current after it has been interrupted, because of the high breakdown voltage of the superficial oxide film on the electrode. Further, the hydrogen overvoltage of these valve metals is higher than that of platinum or gold. Because of this fact, copper, whose dissolution or deposition voltage is lower than that of silver and whose electrolytic potential lies closer to the hydrogen potential, may be used as a plating metal. Also, such a metal as lead may be used with a slight sacrifice in efficiency. It is economically advantageous to substitute such a valve metal as molybdenum, titanium, tungsten or tantalum for platinum or gold and such a metal as copper or lead for silver.

It has been mentioned earlier that such a metal as molybdenum, titanium, tungsten or tantalum permits the electrodeposition of such a metal as silver or copper in spite of the presence of an oxide film on the surface of the former. Careful experiments have revealed that the deposition characteristic and the current interruption characteristic differ with different combinations of metals for the electrode pair. It seems that these characteristics depend upon the valve metal rather than the metal electrochemically dissolved and deposited. It also seams that these characteristics are opposite to each other. FIG. 1 shows typical examples of the manner of the change of a cell voltage and a cell current. With molybdenum and titanium, the deposition characteristics is superior, but the current remaining after the cell voltage has risen is relatively high as shown in graph (a) in FIG. 1. On the other hand, with tantalum the remaining current is very low but the deposition characteristic is inferior as shown in graph (b) in FIG. 1. It is sometimes observed that the deposited metal peels off and, when the peeled metal touches the electrode, the electrode suddenly becomes active to degrade the current interruption characteristic. The plots in FIG. 1 were obtained with experiments conducted by setting the current density for the valve metal electrode to I mA/cm at the time of re-dissolution as well as at the time of depositing. The valve metal electrode in the form of a wire 0.8 mm in diameter, which was sealed with glass to leave its end portion of a length of 4 mm immersed in an aqueous solution containing 0.2 g/l of borax, was subjected to anodic oxidationv by using a platinum plate as the opposite electrode with a source voltage of 50 volts for 30 minutes, and then it was interposed between two plates of copper or silver for the electro-deposition of copper or silver. For the copper electrode, an electrolyte having a composition of 350 g/l Cu( BF,,) 50 g/l HBF,, 5 g/l potassium sodium tartrate (Rochelle salt), 1 g/l p-phenol sulfonic acid and 50 g/l methanol was used. For the silver electrode, an electrolyte having a composition of 90 g/l Ag;,(PO,) and 900 g/l H PO was used. Among the valve metals, tungsten is most recommendable as it is of an excellent deposition nature and provides comparatively a low remaining current.

Even with the tungsten electrode, when it is not sealed with glass, the current interruption characteristic was observed to become inferior to reduce efficiency with repeated deposition and re-dissolution. This is because of the fact that without the glass seal the deposited metal grows over a portion which is apart from the tungsten electrode and during the re-dissolution the deposited metal in the vicinity of the tungsten electrode is dissolved away earlier than the grown metal on the said portion to cut the electrical connection between the grown metal and the tungsten electrode.

FIG. 2 shows the ratio between the dissolved and deposited quantities of copper or silver when the tungsten electrode is sealed in hard glass (a), ceramics (b) and polyvinyl chloride (c) versus the number of cycles of deposition and re-dissolution. As is seen, the reproducibility is most excellent when the electrode is sealed with hard glass. The plots in this Figure are obtained with the same experimental conditions as for FIG. 1.

The coefficient of thermal expansion of hard glass is preferably close to that of tungsten, namely, about 38 X The hard glass is preferably composed of 65 to 75 percent by weight of SiO 5 to 12 percent by weight of B 0 1 to 5 percent by weight of M 0 less than 0.5 percent by weight of Fe O 5 to 8 percent by weight of CaO, 0.5 to 2 percent by weight of MgO, less than 5 percent by weight of ZnO, 6 to 14 percent by weight of Na O and I to 6 percent by weight of K 0.

The composition of the electrolyte will now be discussed. It is readily inferable that, by increasing the concentration of ions of the dissolved metal in the electrolyte, the usable temperature range of the electrolyte may be increased owing to the decrease of the freezing point and increase of the boiling point. However, it sometimes happens that the current interruption characteristic is degradated with an increase of the concentration of Cu or Ag ions beyond a certain limit, although the reason why this occurs is not clear. The limit is about mol/l in the case of Cu ions and about 60 mol/l in the case of Ag ions with the pH value of 2 of the electrolyte, with which it is thought that no insoluble oxide film is formed on the valve metal electrode. The limit varies with the pH value of the electrolyte as shown in FIG. 3. Accordingly, it is thought to be preferable to increase the concentration of ions of the dissolved metal by lowering the pH value to an extent free from the generation of hydrogen at the time of electrodepositing. The lower limit of the pH value seems to be 0 for copper and l for silver. The efficiency of dissolution and deposition begins to decrease when the electrolyte temperature is reduced to a certain value, 'for instance -5 C in the case of a salt of copper and -20 C in the case of a salt of silver. This is thought to stem from the fact that the hydration water contained before the freezing of the liquid electrolyte and deprived of their mobility when the electrolyte temperature is reduced. It is observed that the diffusion becomes extremely slow with the fall in temperature. Accordingly, the effect of the existence of compounds forming complex compounds with Cu or Ag was investigated. FIG. 4 shows the effect of potassium sodium tartrate when the dissolved metal is copper. FIG. 5 shows the effect of phosphoric acid when the dissolved metal is silver. These plots were obtained with the current density for the tungsten set to I00 mA/cm When the dissolved metal is copper, the lowering of the lower limit of the electrolyte temperature is not pronounced by the addition of potassium sodium tartrate. FIG. 6 shows the effect of methanol when the dissolved metal is copper. The most effective content of added methanol is about 50 g/l. From the above data, it is preferable that the electrolyte contains 330 to 380 g/l of Cu(BF 20 to g/l of HBF,, 2 to 10 g/l of potassium sodium tartrate Rochelle salt) and 40 to 60 g/l of methanol when the dissolved metal is copper, while it contains 80 to g/l of Ag PO and 700 to 900 g/l of H PO when the dissolved metal is silver. To improve the deposition characteristic, it is preferable to add about 0.2 to 1.0 g/l of p-phenol sulfonic acid as a surface active agent to either of the above electrolytes.

The current density should be lower than 200 mA/cm in the case of depositing copper and lower than mA/cm in the case of depositing silver on the tungsten electrode with an electrolyte of the above compositions. Otherwise, needle-like crystals become pronounced, resulting in reduced service life.

The electrolytic cell according to the present invention may have different numbers of electrodes for various uses.

EMBODIMENT 1 FIG. 7 shows a preferred embodiment of the cell according to the present invention. It is of the two-electrode construction having a tungsten electrode I and a copper electrode 2. In the case of 10 mA rating, the tungsten electrode is in the form of a wire of a diameter of 0.8 mm and is sealed with a glass mount 3 to leave its end portion of a length of 4 mm immersed in the electrolyte. The coefficient of thermal expansion of the glass of the glass mount 3 is close to that of tungsten 38 X 10"). The copper electrode 2 is cylindrical. Its inner diameter is l 1 mm, its thickness 1 mm, and its height 12 mm. The

material of the copper electrode 2 is purified by more than 99.99 percent. Numeral 4 designates a lid of an electrolyte-resistant resin such as polyvinyl chloride, polystyrene, etc. It is screwed in or bonded to a cell 5 of the same material. Numeral 6 designates a lead. The copper electrode 2 may also serve as a cell as shown in FIG. 8. The relation between the quantity of electricity transferred and the distance between the electrodes (the inner diameter of the copper electrode) is most important in the cell construction. FIG. 9 shows the relation between the quantity of electricity and the number of cycles which has been obtained with the construction shown in FIG. 7.

The element of the two-electrode construction described above is suitable for the storage and reading-out of a given quantity of electricity. When a given quantity of electricity has been stored into this cell by maintaining the tungsten electrode 1 negative, the cell voltage will be suddenly increased to interrupt the cell current if the same quantity of electricity is transferred in the opposite direction. The storage of a given quantity of electricity is effected even if the cell current is not constant during the storing process, because the cell current is integrated through the storing process. Thus, this element is suitable for use in a meter for commercial power supply, gas and water supplies, the battery charger controls, and various timers. FIG. 10 shows a power supply meter using this element. In the Figure, numeral 11 designates a sensor to convert the electric power or flow rate into the corresponding electric current. In the case of a wattmeter, the sensor 11 may be a Hall element for generating a voltage proportional to power or an electrochemical multiplier (solion multiplier) for generating a current proportional to power. In case of a water gauge, the pressure difference across a Venturi orifice or a Pitot tube is converted into the corresponding electric current by utilizing a piezoelectric effect, by means of an electrochemical transducer (solion" transducer) or by utilizing electroosmosis effect. In the case of a wattmeter, a current proportional to the supply current may be caused to pass through the electric quantity memory element 12 with the provision of a shunt resistor 14. This is possible because of a substantially constant supply voltage. There are two methods of reading-out of the meter or gauge. In one method, as shown in FIG. 12, a constant current source is turned on in response to a reading-out start signal to cause a current to flow in the direction opposite to the direction of the current during the storing process until a rise in the cell voltage occurs, which time interval is measured to give the result. In the other method, a predetermined quantity of the electrodeposition of a metal is prepared beforehand on one of the two elements connected in parallel with and in opposite polarities to each other and then the current is reversed to re-dissolve the deposited metal. When the deposited metal is completely redissolved, the cell voltage suddenly rises, whereupon one count of the counting is made, and at the same time the current is reversed again. As the two elements are connected in opposite polarities to each other, the same quantity of the deposited metal as beforehand deposited on one element is deposited on the other element when the previously deposited metal is completely re-dissolved. Thus, by counting the alter nating cycles, the metering during that period may be effected. With a three-electrode cell construction having two tungsten electrodes and third electrode of the electrodeposition metal with a predetermined quantity of the electrodeposition metal beforehand deposited on one of the tungsten electrodes it is able to provide the alternation of a predetermined quantity of electricity between the two tungsten electrodes just in the same manner as with the two-element arrangement as described above. This type of three-electrode construction may thus be advantageously used to carry out the latter reading-out method as described above. In FIGS. 10 to 12, nu-

meral I3 designates an amplifier, numeral 15 a resistor, numeral 16 a diode and numeral 17 a resistor of a high resistance.

Now. the application of the electric quantity memory element according to the present invention to the battery charger will be described.

Though such a battery as a lead battery or a Ni-Cd alkali battery can be charged efficiently up to a point near its saturation point, if this point is passed, the charging efficiency is extremely decreased, and this is accompanied by undesirable gas generation. The charging current can be controlled with the electric quantity control element according to the present invention. Since, if the quantity of electricity discharged in the element is stored, the voltage of the element suddenly rises when the charged quantity becomes equal to the dischargedquantity. An example of the circuit arrangement to this end is shown in FIG. 13. In FIG. 13, numeral 21 designates the electric quantity memory element according to the present invention. During the discharging process of the battery 22 a current proportional to the load current passes through the element 21 to deposite copper or silver on the tungsten electrode. During the charging process a current passes through the element 21 in the opposite direction to dissolve the deposited metal. Upon the completion of the dissolving the voltage of the element 21 suddenly rises to trigger a silicon controlled rectifying element 23 so as to cause a current to flow through a resistor 24. As a result, the voltage drop across the resistor 24 is increased to cut off n-p-n transistor 25, thereupon starting the supplementary charging of the battery with a slight current passing through a resistor 26. With the arrangement described above it is of course possible to charge the battery in spite of the presence of the load (as in the charging of the floating battery). In the Figure, numerals 26, 28, 29 and 30 designate resistors, numeral 27 a shunt resistor, and numeral 31 a diode.

Where the electric quantity memory element is applied to a timer, the setting of time ranging from one minute up to one year is possible by appropriately selecting the quantity of metal to be deposited and the value of the constant current.

EMBODIMENT 2 FIG. 14 shows an example of three-electrode cell according to the present invention. The three-electrode cell may have either a combination of two tungsten electrodes and one copper electrode or a combination of one tungsten electrode and two copper electrodes. As described already, the cell having two tungsten electrodes as shown in FIG. 14 is suitable for use in the alternation of a constant quantity of electricity. The illustrated embodiment comprises two tungsten electrodes 1 and l of the same size and shape and a copper electrode 2. The tungsten electrodes 1 and l are sealed in the same manner as in the embodiment of FIG. 7. The copper electrode 2 is cylindrical with an inner diameter of l 1 mm, a wall thickness of 1 mm and a height of [2 mm. It is disposed concentrically with the tungsten electrodes 1 and l. Lids 4 and 4 carrying the respective tungsten electrodes 1 and l are screwed into or bonded to the copper electrode 2 on opposite ends thereof. If the quantity of electricity involved is small, the copper electrode 2 may also serve as the cell shown in FIG. 15.

The cell having one tungsten electrode and two copper electrodes is suitable for use, for instance, in the integration of electric signals above or below a threshold value. It can also be used for the addition and subtraction of two signals. FIG. I6 shows the construction of this cell. It comprises a tungsten electrode 1 and two copper electrodes 2 and 2'. The tungsten electrode 1 is sealed in the same manner as in the embodiment of FIGS. 7 and 14. It is substantially concentric with the copper electrodes 2 and 2'. The copper electrodes 2 and 2' are cylindrical with an inner diameter of l 1 mm, a wall thickness of I mm and a height of 5 mm. They are spaced by a distance of at least 6 mm from each other. In the case of performing the addition, the two copper electrodes 2 may be the anodes. In the case of performing the subtraction, the copper electrode for a larger signal may be the anode and the copper electrode for a smaller signal may be the cathode. In the case of integrating signals above a threshold value, the element is connected as shown in FIG. 170, while in the case of integrating signals below a threshold value it is connected as shown in FIG. 17b. A constant current equal to the threshold value is caused to flow through one of the copper electrodes.

Although copper is used as the electrode-position metal in the embodiments l and 2, it may be replaced by silver to provide substantially the same functions. In this case, the composition of the electrolyte should be suitably modified and the size of the tungsten electrode should be slightly increased.

We claim:

l. A repeatedly usable memory element for storing a quantity of electricity, comprising:

a first electrode made of a metal electrochemically dissolvable and depositable in accordance with Faradays law;

a second electrode made of a metal selected from the group consisting of tungsten, molybdenum and titanium and having a quantity of the metal constituting said first electrode deposited thereon; and

a liquid electrolyte contacting both said electrodes and containing a soluble salt of the metal constituting said first electrode;

wherein the quantity of electricity which flows from said first electrode to said second electrode is memorized in terms of the quantity of said metal constituting said first electrode that is deposited on said second electrode; and

wherein a change of voltage or current between the two electrodes occurs when the quantity of electricity corresponding to the quantity of said metal deposited on said second electrode has been transmitted from said second electrode to said first electrode.

2. A repeatedly usable memory element according to claim 1, wherein said second electrode consists essentially of tungsten.

3. A repeatedly usable memory element according to claim I, wherein said second electrode is sealed with hard glass having a coefficient of thermal expansion substantially equal to that of said metal constituting said second electrode leaving a portion of said second electrode naked, said portion of said second electrode being immersed in said liquid electrolyte.

4. A repeatedly usable memory element according to claim 1, wherein said first electrode is made of copper and said electrolyte consists of an aqueous solution containing 330 to 380 g/l of Cu( 8H),, 20 to 60 g/l of HBF, and 2 to 10 g/l of potassium sodium tartrate.

5. A repeatedly usable memory element according to claim 4, wherein said electrolyte further contains 40 to 60 g/l of methanol.

6. A repeatedly usable memory element according to claim 4, wherein said electrolyte further contains 0.2 to 1.0 g/l of pphenol sulfonic acid.

7. A repeatedly usable memory element according to claim 1, wherein said first electrode is made of silver and said'electrolyte consists of an aqueous solution containing to g/l of Ag PQ and 700 to 900 g/l of H PO 8. A repeatedly usable memory element according to claim 7, wherein said electrolyte further contains 0.2 to L0 g/l of pphenol sulfonic acid.

9. A repeatedly usable memory element according to claim 1, wherein said second electrode is in the form of a rod and is concentrically surrounded by said first electrode. 

2. A repeatedly usable memory element according to claim 1, wherein said second electrode consists essentially of tungsten.
 3. A repeatedly usable memory element according to claim 1, wherein said second electrode is sealed with hard glass having a coefficient of thermal expansion substantially equal to that of said metal constituting said second electrode leaving a portion of said second electrode naked, said portion of said second electrode being immersed in said liquid electrolyte.
 4. A repeatedly usable memory element according to claim 1, wherein said first electrode is made of copper and said electrolyte consists of an aqueous solution containing 330 to 380 g/l of Cu(BF4)2, 20 to 60 g/l of HBF4 and 2 to 10 g/l of potassium sodium tartrate.
 5. A repeatedly usable memory element according to claim 4, wherein said electrolyte further contains 40 to 60 g/l of methanol.
 6. A repeatedly usable memory element according to claim 4, wherein said electrolyte further contains 0.2 to 1.0 g/l of p-phenol sulfonic acid.
 7. A repeatedly usable memory element according to claim 1, wherein said first electrode is made of silver and said electrolyte consists of an aqueous solution containing 80 to 120 g/l of Ag3PO4 and 700 to 900 g/l of H3PO4.
 8. A repeatedly usable memory element according to claim 7, wherein said electrolyte further contains 0.2 to 1.0 g/l of p-phenol sulfonic acid.
 9. A repeatedly usable memory element according to claim 1, wherein said second electrode is in the form of a rod and is concentrically surrounded by said first electrode. 